Molecular Genetics

Great variation in replicon size and rates of fork movement exists.

Facts

Replicon sizes		Replication Fork Rates		S phase length
Frog		Crepis capillaris	27 Kbp/h	Drosophila
cleavage embryo	3-4 Kbp	Mustard		adult	10 h
adult	30 Kbp	leaf meristem	3.1 Kbp/h	early embryo	4 min
Crepis capillaris	71 Kbp	floral meristem	4.6 Kbp/h	Schizosaccharomyces	10 min
Mustard		Drosophila		Mouse ear skin	35 h
leaf meristem	44 Kbp	most tissues	18 Kbp/h
floral meristem	22 Kbp	brain	62 Kbp/h	DNA is synthesized during only part of the cell division cycle. This part is called the S phase.
Drosophila		General
mean-tissues	137 Kbp	slow fork	18 Kbp/h
brain	91 Kbp	fast fork	350 Kbp/h
General eukaryote	45 Kbp	E. coli	2.2 Mbp/h
Saccharomyces		Fork rates were measured principally by DNA fiber auto-radiography. Conversion constant = 0.34 µm/Kbp. The E. coli rate was calculated from the chromosome synthesis time and the replicon size.
small	13 Kbp
average	160 Kbp
large	900 Kbp

• Replicon sizes can be estimated by electron microsocopy and by sucrose density gradient centrifugation of alkali denatured replicating DNA. In sucrose gradients, newly synthesized single strands of DNA are detected in two size classes: one corresponding to the size of replicons, the other to that of replicon clusters.

Interpretations

• The rate of movement of replication forks in eukaryotes is much slower than in E. coli.
• The eukaryotic rate is variable, depending not only on species but also on tissue and developmental stage.
• Replicon sizes also vary with tissue and developmental stage (ref).
• Given the sizes of replicons and the rate of fork movement, the times calculated for completion of DNA synthesis in a replicon approach the length of S phases.
• Conversely, the length of S phase in any tissue is determined by replicon spacing, number and fork rate (ref).
• Without multiple replicons (multiple origins of replication), the replication of eukaryotic genomes would take a long time.

Further information

• Replicon clusters are organized in families.

This is page 13131 of Molecular Genetics by Ulrich Melcher, © 1997-9, 2003